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https://github.com/alangpierce/go-forceexport

A golang package that allows you to access unexported functions from other packages
https://github.com/alangpierce/go-forceexport

Last synced: 17 days ago
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A golang package that allows you to access unexported functions from other packages

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README 

        
# go-forceexport



go-forceexport is a golang package that allows access to any module-level

function, even ones that are not exported. You give it the string name of a

function , like `"time.now"`, and gives you a function value that calls that

function. More generally, it can be used to achieve something like reflection on

top-level functions, whereas the `reflect` package only lets you access methods

by name.



As you might expect, this library is **unsafe** and **fragile** and probably

shouldn't be used in production. See "Use cases and pitfalls" below.



It has only been tested on Mac OS X with Go 1.6. If you find that it works or

breaks on other platforms, feel free to submit a pull request with a fix and/or

an update to this paragraph.



## Installation



`$ go get github.com/alangpierce/go-forceexport`



## Usage



Here's how you can grab the `time.now` function, defined as

`func now() (sec int64, nsec int32)`



```go

var timeNow func() (int64, int32)

err := forceexport.GetFunc(&timeNow, "time.now")

if err != nil {

    // Handle errors if you care about name possibly being invalid.

}

// Calls the actual time.now function.

sec, nsec := timeNow()

```



The string you give should be the fully-qualified name. For example, here's

`GetFunc` getting itself.



```go

var getFunc func(interface{}, string) error

GetFunc(&getFunc, "github.com/alangpierce/go-forceexport.GetFunc")

```



## Use cases and pitfalls



This library is most useful for development and hack projects. For example, you

might use it to track down why the standard library isn't behaving as you

expect, or you might use it to try out a standard library function to see if it

works, then later factor the code to be less fragile. You could also try using

it in production; just make sure you're aware of the risks.



There are lots of things to watch out for and ways to shoot yourself in

the foot:

* If you define the wrong function type, you'll get a function with undefined

  behavior that will likely cause a runtime panic. The library makes no attempt

  to warn you in this case.

* Calling unexported functions is inherently fragile because the function won't

  have any stability guarantees.

* The implementation relies on the details of internal Go data structures, so

  later versions of Go might break this library.

* Since the compiler doesn't expect unexported symbols to be used, it might not

  create them at all, for example due to inlining or dead code analysis. This

  means that functions may not show up like you expect, and new versions of the

  compiler may cause functions to suddenly disappear.

* If the function you want to use relies on unexported types, you won't be able

  to trivially use it. However, you can sometimes work around this by defining

  equivalent copies of those types that you can use, but that approach has its

  own set of dangers.



## How it works



The [code](/forceexport.go) is pretty short, so you could just read it, but

here's a friendlier explanation:



The code uses the `go:linkname` compiler directive to get access to the

`runtime.firstmoduledata` symbol, which is an internal data structure created by

the linker that's used by functions like `runtime.FuncForPC`. (Using

`go:linkname` is an alternate way to access unexported functions/values, but it

has other gotchas and can't be used dynamically.)



Similar to the implementation of `runtime.FuncForPC`, the code walks the

function definitions until it finds one with a matching name, then gets its code

pointer.



From there, it creates a function object from the code pointer by calling

`reflect.MakeFunc` and using `unsafe.Pointer` to swap out the function object's

code pointer with the desired one.



Needless to say, it's a scary hack, but it seems to work!



## License



MIT